1. Field of the Invention
The present invention relates to a method for driving such display devices as a liquid crystal display device, thin film EL display device, and the like.
2. Description of the Prior Art
For a liquid crystal display device and the like, a frame thinning out system is well-known as a driving method for enabling such a display panel to display a gradationally toned image, with several brightness levels, on its screen.
The frame thinning out system turns on pixels only in a number of frames corresponding to a level of brightness to be displayed out of all frames within an integration time period consisting of a plurality of frame time periods, so as to visually obtain a medium level of brightness in each integration time period.
The applicant of the invention has already proposed a driving method to reduce flicker on the screen by grouping all pixels of the display device into groups, each of which is composed of a plurality of pixels contiguous to each other and limiting the total number of pixels to be turned on for each frame time period within the same group according to its display data as a display driving method, improving the conventional frame thinning out system.
Table 1 shows gradation rules (rules of turning pixels on or off for which frame time period) in case of making a gradational display by setting eight frame time periods as one integration time period by means of this driving method. In this case, all pixels on the screen are grouped into groups, each of which is composed of four pixels contiguous to each other as shown by arrow II, on the display panel composed of a plurality of pixels arranged in the form of matrix on it as shown schematically in the plan view of FIG. 1.
In Table 1, numeral 1 indicates turning a pixel on and numeral 0 indicates turning a pixel off.
FIG. 2 is a plan view schematically showing a magnification of one group of pixels shown by arrow II in the display panel. In the grouping, when setting the pixel in the upper left hand corner of the screen of the display panel 1 as a pixel in the 0th row and 0th column, the pixel arrangement is set so that in FIG. 2 the two upper pixels A and B are in an even-numbered row, the two lower pixels C
TABLE 1 ______________________________________ Gradation Frame level Pixel 1 2 3 4 5 6 7 8 ______________________________________ 0/8 A 0 0 0 0 0 0 0 0 B 0 0 0 0 0 0 0 0 C 0 0 0 0 0 0 0 0 D 0 0 0 0 0 0 0 0 1/8 A 1 0 0 0 0 0 0 0 B 0 0 1 0 0 0 0 0 C 0 0 0 0 1 0 0 0 D 0 0 0 0 0 0 1 0 2/8 A 1 0 0 0 1 0 0 0 B 0 1 0 0 0 1 0 0 C 0 0 1 0 0 0 1 0 D 0 0 0 1 0 0 0 1 3/8 A 1 0 0 1 0 0 0 1 B 0 1 0 0 0 1 1 0 C 0 1 1 0 0 1 0 0 D 0 0 0 1 1 0 0 1 4/8 A 1 0 1 0 1 0 1 0 B 0 1 0 1 0 1 0 1 C 1 0 1 0 1 0 1 0 D 0 1 0 1 0 1 0 1 6/8 A 0 1 1 1 0 1 1 1 B 1 0 1 1 1 0 1 1 C 1 1 0 1 1 1 0 1 D 1 1 1 0 1 1 1 0 7/8 A 0 1 1 1 1 1 1 1 B 1 1 0 1 1 1 1 1 C 1 1 1 1 0 1 1 1 D 1 1 1 1 1 1 0 1 8/8 A 1 1 1 1 1 1 1 1 B 1 1 1 1 1 1 1 1 C 1 1 1 1 1 1 1 1 D 1 1 1 1 1 1 1 1 ______________________________________
and D are in an odd-numbered row, the two left pixels A and C are in an even-numbered column, and the two right pixels B and D are in an odd-numbered column.
FIG. 3 is a schematic diagram showing a gradation rule of each of pixels A to D within one group in case of displaying the 6/8 gradation level all over the screen of the display panel 1 according to the gradation rule.
As known from FIG. 3, in the case of displaying the 6/8 gradation level, three pixels B, C and D are turned on (the pixels A is turned off) for the first frame time period, three pixels A, C and D are turned on (the pixel B is off) for the second frame time period, three pixels A, B and D are turned on (the pixel C is off) for the third frame time period, and three pixels A, B and C are turned on for the fourth frame time period. The rules for the first to fourth frame time periods are also applied to the fifth to eighth frame time periods, respectively, and after this, the cycle is repeated for eight frame time periods as an integration time period. Namely, in the case of displaying the 6/8 gradation level, three pixels are always turned on for each frame time period and each of pixels A to D are turned on, equally, six times for eight frame time periods.
In this case brightness level as a whole of a unit image area of one group composed of pixels A to D is as shown in FIG. 4.
Namely, since three pixels are turned on throughout all frame time periods, the brightness level is always 3/4. Accordingly, the 6/8 brightness level is displayed, on the average, during one integration time period consisting of eight frame time periods. Further, brightness level of the group does not vary as a whole of the unit image area, so flicker is reduced.
In the above-mentioned driving method previously proposed, however, as shown in FIG. 3, in case of the 6/8 brightness level, the number of pixels turned on in a subgroup of pixels A and B in the upper even-numbered row out of the pixels A to D within one group for each frame time period is different from the number of pixels turned on in a subgroup of pixels C and D in the lower odd-numbered row. As a result, there has been a problem that quality of the display is deteriorated depending on a viewing angle in the case where the above-mentioned driving method is applied to such a capacitive display device as a liquid crystal display device.
Namely, since such a capacitive display device as a liquid crystal display device shows a phenomenon that brightness of the pixel turned on at that time varies according to the number of pixels turned on out of the pixels arranged in one row (common line) on the screen of the display panel 1. Namely the difference between the number of pixels turned on in a subgroup of pixels A and B in the upper even-numbered row out of the pixels A to D within one group and the number of pixels turned on in a subgroup of pixels C and D in the lower odd-numbered row causes such deterioration of quality of the display that brightness of the pixels vary for each frame time period, and the bright points appear to be vibrating.
This problem also occurs in the case of displaying the other gradation levels.